The present invention relates to zinc-nickel electroplated articles exhibiting corrosion resistance, including articles that, after being plated with a corrosion-resistant, zinc-nickel alloy, exhibit low hydrogen embrittlement characteristics.
High-strength steels are subject to delayed, brittle failures at relatively low stresses. Such failures have been attributed to the presence of hydrogen in the steel microstructure. The hydrogen can be introduced into the microstructure by reaction with water or with an acid, or most importantly, by cathodically discharging hydrogen at the surface of the steel. Since most high-strength steels have corrosion-resistant coatings that are applied by less than 100 percent efficient electroplating techniques, hydrogen is discharged onto the steel surface along with the corrosion-resistant coating. Therefore, the quantity of hydrogen deposited at the coating steel interface must be carefully monitored and controlled.
Currently, a cadmium-titanium alloy is electroplated onto high-strength steels under carefully controlled conditions. The resulting plated product is then heat treated at elevated temperatures to achieve an acceptable low hydrogen embrittlement level. It is believed that the porosity of the electroplated cadmium-titanium alloy is the key to the removal of the hydrogen during a subsequent heat treatment operation; however, the cadmium-titanium plating bath is very sensitive to contamination, which can cause embrittlement characteristics in coated high-strength steel substrates. More importantly, however, the cadmium-titanium alloy plating bath contains both cadmium and cyanide, which create disposal problems unless expensive waste treatment equipment is employed.
Zinc-nickel alloys have been suggested for electroplating onto steels to render them corrosion-resistant. The plating parameters of a zinc-nickel alloy plating bath are much easier to control and maintain than a cadmium-titanium bath. Additionally, the zinc-nickel bath is cadmium and cyanide free and contains components that are relatively nontoxic compared to those in a cadmium-titanium plating bath. Most prior zinc-nickel baths have, however, been suggested only for high-speed plating. High-speed plating techniques are unsatisfactory for rack plating of larger metal parts. Although it has been suggested that a zinc-nickel bath can be plated at low and medium current densities by employing an ammonium chloride electrolyte, prior attempts at plating at low current densities have led to pitted and spongy deposits that do not provide good corrosion resistance. In addition, these prior attempts to produce a zinc-nickel corrosion-resistant coating at low current densities have led to a relatively high degree of hydrogen embrittlement in plated high-strength steel parts to which the coating has been applied.